1. Field of the Invention
The present invention relates generally to a ghost cancelling system, and is directed more particularly to a ghost cancelling system by which a ghost signal is eliminated at the video signal stage.
2. Description of the Prior Art
In the art, there is known such a system which eliminates a ghost signal at the video stage as shown in FIG. 1. In the prior art system shown in FIG. 1, the signal received by an antenna 1 is fed through a tuner 2 and a VIF (video intermediate frequency) amplifier 3 to a video detector circuit 4 wherein the video signal is detected. The video signal therefrom is fed to a subtraction type adder 5 which is also supplied with the ghost cancelling signal which is formed by imitating the input ghost signal from a transversal filter as will be described later. Thus, the video signal from which the ghost signal is eliminated is derived from the adder 5 and then delivered to an output terminal 6.
The video signal delivered from the adder 5 is fed through a contact 36a of a mode selection switch 36 to a delay circuit 7 which forms a part of a transversal filter. This delay circuit 7 is formed of a plurality of delay units, each of which has a delay time corresponding to a signal sampling period (for example, 100 nano-seconds), connected at a plurality of stages, and n's taps are respectively led out from the respective stages. The respective outputs from the taps are fed to weighting function circuits 10.sub.1, 10.sub.2, -10.sub.n of the multiplier type.
The video signal from the adder 5 is fed also to a differentiation circuit 11 by which the ghost level is detected. In this case, the level detecting period of the ghost is selected to be such a period which is contained in the standard television signal and is not affected by the video signal as long as possible, for example, the period of the vertical synchronizing signal is generally used as the above ghost detecting period. In general, as shown in FIG. 2, the period of H/2 from a front edge VE of a vertical synchronizing signal to an equalizing pulse HE is selected as the detecting period. The signal level during the detecting period is differentiated and the tapped outputs of the delay units are weighted in proportion to the differentiated level.
For example, in such a case where a ghost signal with a phase difference .phi.(.phi.=.omega..sub.c .tau., .omega..sub.c is an angular frequency of a video carrier at a high frequency stage) from the video signal of 45.degree. and a delay time .tau. at the RF stage of 45.degree. is contained, there appears such a video signal with the waveform shown in FIG. 3A in the ghost detecting period. This signal is differentiated and inverted in its polarity. Then, a signal with the differentiated waveform shown in FIG. 3B is obtained. This differentiated waveform can be approximately regarded as the impulse response of the ghost signal.
The differentiated waveform, which corresponds to the ghost level detecting signal, delivered from the differentiation circuit 11 is supplied through an amplifier 37 to a demultiplexer 12. This demultiplexer 12 is formed of a plurality of delay units, each of which has the delay time corresponding to the signal sampling period, connected at a plurality of stages, and n's taps are led out from the respective stages similar to the delay circuit 7. The respective tapped outputs are fed to switching circuits 15.sub.1, 15.sub.2, -15.sub.n, respectively.
The video signal from the video detector circuit 4 is also fed to a sync. separator circuit 16 which supplies its output to a gate pulse generator 38. This gate pulse generator 38 produces gate pulses, each of which corresponds to the period of H/2 from the front edge VE of the vertical synchronizing signal, and supplies the same to the switching circuits 15.sub.1 to 15.sub.n to render them ON.
The signals from the switching circuits 15.sub.1 to 15.sub.n are respectively fed to analog accumulative adders respectively consisting of resistors 17.sub.1, 17.sub.2, -17.sub.n and capacitors 18.sub.1, 18.sub.2, -18.sub.n. The signals from the capacitors 18.sub.1 to 18.sub.n are fed to the weighting function circuits 10.sub.1 to 10.sub.n, respectively. The outputs from the weighting circuits 10.sub.1 to 10.sub.n are fed to an adder circuit 20 and then added therein to produce a ghost cancelling signal which is then fed to the subtraction type adder 5.
The above delay circuit 7, weighting circuits 10.sub.1 to 10.sub.n and adder circuit 20 form a so-called transversal filter. This transversal filter is inserted into the feedback loop, so that a so-called inverse filter is formed and hence the ghost signal component contained in the input video signal can be eliminated. In this case, the distortion of a waveform within a H/2 period from the front edge of a certain vertical synchronizing signal is detected and the weighting functions are determined. Thereafter, if the ghost signal component still remains uncancelled in the output video signal, the distortion is, further detected by the differentiation circuit 11 to reduce the ghost component which remain uncancelled in the output video signal and the accumulative adder operates therefor.
When the mode selection switch 36 is changed over to its terminal 36b, the ghost cancelling circuit is changed over to the feed-forward mode from the feedback mode. Even in the feed-forward mode, the ghost cancelling operation similar to that described as above is carried out.
FIG. 4 shows another type of the prior art ghost cancelling system. In the example of FIG. 1, there is employed the transversal filter of the output adding type in which the tapped outputs from the delay circuit 7 are weighted and then added at the adder circuit 20 to produce the ghost cancelling signal. While, in the example of FIG. 4, such a transversal filter of the input adding type is employed in which input signals which are weighted are respectively fed to the taps of the delay circuit 7. The ghost cancelling operation of the example shown in FIG. 4 is entirely same as that of FIG. 1.
In either of the prior art examples shown in FIGS. 1 and 4, individually independent two delay circuits such as the demultiplexer 12 for sampling the waveform of the input video signal in the ghost detecting period and generating the weighting signal necessary to imitate the ghost component and the delay circuit 7 for generating the ghost cancelling signal are necessary. Therefore, the prior art systems become bulky and complicated in construction. Further, when the delay characteristics of both delay circuits 7 and 12 are not equal, the ghost cancelling operation becomes incomplete. Further, when the entire ghost cancelling circuits are made an IC, since the number of external terminals to led out is many and so on, the respective delay circuits occupy a large area of a semiconductor wafer. As a result, it is difficult to provide two delay circuits as an IC of one tip, or that is, it is necessary to divide the IC as two tips.